added item_21

Author: craigem

item_21.py

@@ -0,0 +1,106 @@
+#!/usr/bin/env python3
+
+'''Item 21 from Effective Python'''
+
+
+# Example 1
+''' ignore OverflowError exceptions and return zero instead '''
+print('Example 1:\n==========')
+
+def safe_division(number, divisor, ignore_overflow, ignore_zero_division):
+    try:
+        return number / divisor
+    except OverflowError:
+        if ignore_overflow:
+            return 0
+        else:
+            raise
+    except ZeroDivisionError:
+        if ignore_zero_division:
+            return float('inf')
+        else:
+            raise
+
+
+# Example 2
+''' This call will ignore the float overflow from division and will return zero
+'''
+print('\nExample 2:\n==========')
+
+result = safe_division(1, 10**500, True, False)
+print(result)
+
+
+# Example 3
+''' This call will ignore the error from dividing by zero and will return
+infinity '''
+print('\nExample 3:\n==========')
+
+result = safe_division(1, 0, False, True)
+print(result)
+
+
+# Example 4
+''' One way to improve the readability of this code is to use keyword
+arguments '''
+print('\nExample 4:\n==========')
+
+
+def safe_division_b(number, divisor,
+                  ignore_overflow=False,
+                  ignore_zero_division=False):
+    try:
+        return number / divisor
+    except OverflowError:
+        if ignore_overflow:
+            return 0
+        else:
+            raise
+    except ZeroDivisionError:
+        if ignore_zero_division:
+            return float('inf')
+        else:
+            raise
+
+
+# Example 5
+''' Then callers can use keyword arguments to specify which of the ignore flags
+they want to flip for specific operations, overriding the default behavior '''
+print('\nExample 5:\n==========')
+
+result = safe_division_b(1, 10**500, ignore_overflow=True)
+print(result)
+result = safe_division_b(1, 10**500, ignore_zero_division=True)
+print(result)
+
+
+# Example 6
+''' In Python 3, you can demand clarity by defining your functions with
+keyword-only arguments. These arguments can only be supplied by keyword, never
+by position '''
+print('\nExample 6:\n==========')
+
+
+def safe_division_c(number, divisor, *,
+                    ignore_overflow=False,
+                    ignore_zero_division=False):
+    try:
+        return number / divisor
+    except OverflowError:
+        if ignore_overflow:
+            return 0
+        else:
+            raise
+    except ZeroDivisionError:
+        if ignore_zero_division:
+            return float('inf')
+        else:
+            raise
+
+
+# Example 7
+''' Keyword arguments and their default values work as expected '''
+print('\nExample 7:\n==========')
+
+result = safe_division_c(1, 0, ignore_zero_division=True)
+print(result)